1. Field of the Invention
The present invention relates to a rubber thread for a golf ball (which will be hereinafter referred to as a xe2x80x9crubber threadxe2x80x9d) and a golf ball using the rubber thread.
2. Description of the Related Art
A golf ball to be used for a play in a golf course is roughly divided into a wound golf ball having a core in which a rubber thread is wound and a solid golf ball (a two pieces golf ball, a three pieces golf ball and the like) having a core formed of only a solid rubber. In general, the wound golf ball is excellent in a hit feeling and a control performance and the solid golf ball is excellent in a flight distance and a durability. The wound golf ball has been used for a long time. For a certain period, almost all first-class golf balls were represented by the wound golf balls. However, the solid golf ball developed later can be manufactured easily at a low cost. In recent years, therefore, more solid golf balls than the wound golf balls have been put on the market.
Under the circumstances, a professional golf player and an advanced amateur golf player still require the wound golf ball which is excellent in the hit feeling and the control performance. The professional golf player and the advanced amateur golf player have desired the wound golf ball providing a flight distance equivalent to that of a solid golf ball. Moreover, many general amateur golf players want to use the wound golf ball if the flight distance is increased.
The golf ball is launched and flies by hitting with a golf club. In general, if the initial speed of the golf ball is higher immediately after the hitting, a flight distance tends to be increased. Accordingly, it is possible to lengthen the flight distance of the wound golf ball by increasing the initial speed, that is, a resilience coefficient.
An ordinary wound golf ball includes a core and a cover. The core is constituted by a center (a solid center or a liquid center) and a rubber thread layer having a rubber thread drawn and wound around the center. The rubber thread layer most contributes to the resilience performance of the wound golf ball. An attempt to enhance the resilience performance of the wound golf ball has been made by improving the rubber thread to be used for the rubber thread layer for a long period of time. For example, Japanese Patent Publication No. Sho 61-12706 (1986/12706) has disclosed a wound golf ball in which carbon black is used for a rubber composition constituting a rubber thread. Moreover, Japanese Patent Publication No. Hei 5-41272 (1993/41272) has disclosed a wound golf ball in which a specific base rubber issued for a rubber composition constituting a rubber thread.
The rubber thread layer is formed by winding the rubber thread drawn as described above. Usually, a draw ratio is very high, that is, approximately 800% to 1100%. Therefore, the rubber thread is easily crystallized due to the drawing. If the degree of crystallization is high, the resilience performance of the golf ball becomes insufficient. In order to suppress the crystallization, the following means can be supposed.
(1) to increase a ratio of a polysulfide bond in the rubber thread; and
(2) to use synthetic polyisoprene having a high ratio of a trans 1,4 bond.
The rubber thread is obtained by crosslinking a rubber. The crosslinking is usually carried out by using a sulfur and a vulcanization accelerator together. By devising the type of the vulcanization accelerator, the ratio of the polysulfide bond can be increased. More specifically, a thiazole type vulcanization accelerator, a basic amine type vulcanization accelerator or the like issued. In the case in which the thiazole type vulcanization accelerator and the amine type vulcanization accelerator are used together, the amine type vulcanization accelerator increases the speed of the crosslink carried out by the thiazole type vulcanization accelerator, thereby contributing to an enhancement in a production efficiency.
The cover is obtained by covering the rubber thread layer with a material such as an ionomer resin. At a covering step, the material is softened at a high temperature and the cover is molded. In this case, the rubber thread is also exposed at a high temperature (for example, 140xc2x0 C. or more). In some cases, consequently, the rubber thread causes a thermal deterioration. If the thermal deterioration is great, the resilience performance of the golf ball is degraded in spite of the suppression of the crystallization during the drawing.
In consideration of the above-mentioned problems, it is an object of the present invention to provide a rubber thread for a golf ball which rarely causes the crystallization during the drawing and the thermal deterioration when molding the cover and to provide a golf ball using the rubber thread.
In order to achieve the object, the present invention provides a rubber thread for a golf ball obtained by crosslinking a rubber with a sulfur and a vulcanization accelerator, wherein the rubber contains 60% by weight or more of synthetic polyisoprene having a trans 1,4 bond ratio of 2% to 4%, and the vulcanization accelerator contains 66% by weight or more of a nonbasic vulcanization accelerator.
As described above, an amine type vulcanization accelerator is preferably used for a conventional rubber thread. The present inventor found that a basic vulcanization accelerator such as the amine type vulcanization accelerator causes the promotion of a thermal deterioration and completed the present invention. The nonbasic vulcanization accelerator is used for the rubber thread according to the present. invention and a ratio of the nonbasic vulcanization accelerator to a total vulcanization accelerator is 66% by weight or more (that is, the ratio of the basic vulcanization accelerator is 34% by weight or less). Therefore, the thermal deterioration of the rubber thread can be suppressed. Since the rubber thread is used, the golf ball can have an excellent resilience performance. Although the detailed reason why the basic vulcanization accelerator promotes the thermal deterioration is not clear, it is presumed that a residual functional group of the basic vulcanization accelerator acts as an origin to cut a sulfide bond by heating when molding a cover. It is preferable that a ratio of the nonbasic vulcanization accelerator to a total vulcanization accelerator should be 75% by weight or more.
The amine type vulcanization accelerator also contributes to an enhancement in a polysulfide bond ratio as described above, and furthermore, to the suppression of crystallization during the drawing. In the rubber thread according to the present invention, the amount of the basic vulcanization accelerator (including the amine type vulcanization accelerator) to be blended is controlled. However, the rubber thread contains 60% by weight or more of synthetic polyisoprene having a high trans 1,4 bond ratio, more specifically, syntheticpolyisoprene having a trans 1,4 bond ratio of 2% to 4%. Therefore, the crystallization can also be suppressed sufficiently during the drawing.
It is preferable that the vulcanization accelerator should contain 10% by weight to 30% by weight of the amine type vulcanization accelerator. Consequently, the thermal deterioration can be suppressed when molding the cover, and furthermore, a time required for crosslinking of the rubber thread can be shortened so that the production efficiency of the golf ball can be enhanced. It is preferable that a ratio of the amine type vulcanization accelerator to the total vulcanization accelerator should be 15% by weight to 30% by weight.
It is preferable that a weight ratio of an amount of the sulfur to that of the vulcanization accelerator, that is, a value of (the amount of the sulfur/the amount of the vulcanization accelerator) should be 0.8/1 to 8/1. Consequently, the polysulfide bond ratio in the rubber thread can be maintained almost completely in spite of the control of the amount of the basic vulcanization accelerator to be blended.
The rubber thread is particularly suitable for the golf ball in which the molding temperature of the cover is 140xc2x0 C. or more. At a step of molding the cover at a high temperature (140xc2x0 C. or more) which might cause the thermal deterioration, it is possible to remarkably obtain the effect of suppressing the thermal deterioration by a reduction in the amount of the basic vulcanization accelerator to be blended.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.